Electro Magnetic Clutch System
Introduction
A clutch is a machine component used for the transmission of power from a driving shaft to a driven shaft that is to be started and stopped frequently. The application of clutches is also found in many cases where the power to be transmitted is in partial or full load conditions.In recent years, electromagnetic clutch system is employed in many applications including automotive sector. The electromagnetic clutch system is used to transmit, interrupt or decelerate and stop the torque by the electromagnetic action.The main difference between the normal mechanical clutch and the electromagnetic clutch is the control of movement of the pressure plates. In case of mechanical clutch a spring is used for the engagement and disengagement of the clutch whereas in electromagnetic clutch electromagnetic field is applied for the engagement and disengagement.
Construction and Working Principle
There are four major components used in electromagnetic clutch system namely (I) field, (ii) rotor, and (iii) huband (iv) armature.
Field or Field Coil
This is also called as a coil shell. It comprises a copper wire formed as a coil cast in synthetic resin such as epoxy coating and is fixed to the inner part of a shell. This shell is made of carbon steel.An aluminium wire can also be used as a coil. The coil is placed behind a rotor.
Rotor
It is connected to a driving shaft or input shaft of the machine from which the power is to be transmitted.
Hub
It is connected to a driven shaft or output shaft which receives the power from the input shaft.
Armature
It is mounted to the hub of the driven shaft and is coated with a friction liner.
An air gap is provided between the armature and the rotor to act as a resistance which must be overcome during engagement of the clutch.
Working Principle
The electromagnetic clutch system works on the electromagnetic principleand transmits the torque mechanically. An electric circuit energizes the coil when the electromagnetic system is activated for the engagement of the clutch. A magnetic field is generated when the current passes through the coil. This magnetic flux overcomes the air gap provided between the armature and the coil shell and pullsin the armature in contact with the rotor for theengagement or acceleration. During this time, there will be a slip between the armature and the rotor till the output shaft attains the speed of input shaft. Normally, the time required for the rotor to attain the full speed is in the range of 0.02 second to 1 second. The matching of the speed between the input and output shafts is sometimes referred to as 100% lockup.Generally, there are two engagement times. The first one is to generate and develop the magnetic field strong enough to pull the armature plate and the second one is time to speed up the clutch. The time to speed up the clutch is purely related to the inertia of the system. The inertia depends largely on the mass and the geometry of the electromagnetic system.
When the electric circuit breaks off the power supply to the coil, the magnetic flux drops down rapidly and the armature plate comes back to its original position by the action of spring force. In most of the electromagnetic system, a spring is provided to hold the armature plate in position and provide a predetermined air gap when the power supply is removed from the system. If the air gap is larger, then it will take longer the armature to develop enough magnetic attractiondurin the engagement of the clutch. In high cycle applications, floating type armatures are used. These armatures rest against the rotor and provide zero air gaps.
The main factor influencing the torque rating is the combination of current and voltage. The amount of magnetic flux can be increased by increasing the size and the number of turns of the copper or aluminium wire. If accurate or maximum torque is required from the clutch then it is always preferred to use a constant power supply source. The magnetic flux will degrade due to the resistance of the coils if the supply power is not constant or varies continuously. If the coil gets heated up beyond the limit then the torque transmitted by the clutch decreases and this will be about 8% for every additional 20oC rise in temperature. From the fundamental of electricity we know that,
V = I R
Where, V = Voltage on Volts
I = Current in amp
R = Resistance in ohms
As a result of increase in temperature of the coil, the resistance increases and subsequently the current drops down. The torque will be same as produced by the electromagnetic system as long as the system is supplied with a right voltage and current during the operation.
Types
There are three types of electromagnetic clutches namely (I) electromagnetic particle clutch which may be of single plate or multiple plates, (ii) electromagnetic hysteresis clutch and (iii) electromagnetic eddy current clutch. The hysteresis and eddy current clutches are of non-contact type.
Applications
Electromagnetic clutch system can be used for any remote applications due to the absence of mechanical linkages. These systems are employed in printing machinery, copying machines, conveyor drives, air conditioning units, automobiles, medical equipment, packaging machinery, material handling equipment, agricultural equipment and many industrial applications.
Advantages
- Electromagnetic clutches provide an efficient and electrically switchable link.
- The application of complicated linkages used for the control mechanism is eliminated.
- The ratio of torque transmitted by the clutch to the size of the clutch is high.
Limitations
- The initial cost electromagnetic clutch system is high as compared to that of mechanical clutch system.
- Dust and contaminations between the contact surfaces reduce the torque.
- The operating temperature is limited since the high temperature insulation gets damaged.
- Temperature of the field coil shoots up rapidly during the engagement of the clutch.
- Periodic cleaning of the brushes is required for better energization.